XMM-Newton
Users Handbook

The RGS wavelength scale is determined by the relative alignment of
the various instruments and their structural stability. Studies of
spectra of emission line sources have shown that line positions are
systematically shifted with respect to laboratory wavelengths. These
shifts have been found to be correlated with the angular distance
between the spacecraft pointing direction and the Sun (``solar
angle''). The correction of this dependence aligns the wavelength
scales of both spectrographs and orders, and improves substantially
their accuracy.
Fig. 82 shows the residuals of the RGS measured
versus nominal positions for certain emission lines and a set of
observations of coronal sources. For an on-axis source, the
wavelength scale accuracy is 5 mÅ and 3 mÅ (1) in first
and second order, respectively, after application of the heliocentric
velocity correction and removal of the solar angle dependence. It has
been verified that the wavelength scale is stable over a long
timescale.

Figure 82:Comparison of measured and laboratory wavelengths
for a number of sources and observations. Observed wavelengths
have been corrected for solar angle dependence and for
heliocentric velocity.